I keep hearing about unknown, but rigorous small liberal arts colleges, but colleges that are known for heavy workload like Swarthmore, Reed, Bowdoin are all highly ranked top LACs. What are some really unknown LACs that are rigorous, on par with colleges listed above?
Rhodes, Furman, Grinnell, Wofford, University of Richmond and Elon were schools I looked at while looking for rigorous LACs.
Mt Holyoke, Weslyan, Bates, Scripps, Pitzer, Hamilton
Allegheny?
I have heard that Franklin and Marshall has a heavy workload and rigor. I would not, however, call it an “unknown,” nor would I consider some of the other names given above to be “unknown.” But I guess that is a relative term!
Lawrence is a school that many think, if located back east, would be much higher ranked. Knox is another one I have been very impressed with, and my kid, who sat in on classes at 15+ LACs, including Vassar, Bates, Oberlin, reported that the most exciting, challenging class he attended was at Knox.
Wooster, Bard, Union, Bryn Mawr, Haverford, Earlham, Davidson, Oberlin, Kenyon, all the NESCAC schools… really, there are so many!
As for unknown, that may depend on where you are in the country. I can pretty much guarantee that Carleton would elicit a blank stare from folks in my neck of the woods yet would not in Minneapolis.
Most of these are actually pretty well known, I don’t follow LACs all that much and I recognized most of these names.
Well, @theloniusmonk , as I tell DS, “most of the people who matter will have heard of your (NESCAC ) school.” So far, using that differentiator, we’ve established you as someone who might matter and pretty much everyone who votes in the building we do as someone who doesn’t! 
New College of Florida is a public (formerly private) LAC that is less selective, lower ranked, and (at full sticker) less expensive than the NESCAC schools. It might not be as well known as the “top” LACs in most of the country, but has long had a pretty solid academic reputation.
https://www.ncf.edu/about/
St. Mary’s College of Maryland is another public honors college you might want to consider.
St. John’s College, with branches in Annapolis and Santa Fe, has an interesting Great Books curriculum. It too is less selective and a bit lower ranked than the NESCAC schools. I would expect based on the course content that it is rigorous, but don’t really know for sure.
The ultimate in small, obscure, and apparently rigorous schools may be Deep Springs College. Its work-study program only lasts 2 years, after which many students transfer to tip-top colleges before moving on to grad school.
https://en.wikipedia.org/wiki/Deep_Springs_College
(I don’t know for a fact that its academic program is any more rigorous than Swarthmore’s or Reed’s, but the cattle-moving part must be challenging if you weren’t raised on that kind of thing.)
Macalester College in St. Paul, MN is another one. I believe it also allows students an opportunity to study bagpipes for free.
Whitman College in Walla Walla, WA. Also Colorado College (the block plan is rigorous by nature) though more well known nationally compared to Whitman.
I also suggest Whitman. And also many of the schools listed above. As @TheGreyKing mentioned, most of these are known by people who need to know, (also by a lot of Cc’ers), and a lot of them are known mostly in their regions. I also think Deep Springs is super interesting, but good luck getting in. I believe it’s more selective than most Ivy League schools, but with only 25 students, you can see why.
Based on the above posts, ‘rigorous’ seems to be a rather elastic concept! The OP seems to link ‘rigorous’ with ‘heavy workload’. I’ve seen too much busy work in my time to be impressed by volume of work- I think a rigorous school is one where the learning expectations are high, and it takes serious, sustained effort to excel.
Probably any of top 50 (?) LAC has the potential to be rigorous- depending a lot on the choices a student makes. But while I know students at quite a few of the schools named above who have not found their schools rigorous, I have never heard a Swattie say that!
Along the lines of what @collegemom3717 said, there are a lot of “good” schools, but…
… there aren’t any you haven’t heard of that are on par with the ones you listed.
Regionally, you should look at the members of the Associated Colleges of the South some of which are not well known. It is a consortium of small and rigorous LACs from VA down to Texas. I have personal knowledge of Centre, Davidson, Furman, Rhodes, Sewanee and W&L. All of those are comparable academically to NESCAC but not as well known outside the South. Graduate Schools and employers know this. It really is a matter of fit. There may be similar consortia in other regions that would make a good starting list. http://colleges.org/colleges/
^^^when will the 2018 USNews best college (including LACs) ranking come out? This month, right? I thought it would be out by now.
Colby in Maine and Carleton in Minn are excellent. Look at Beloit in Wisconsin. University of Dubuque Loras and Clarke in northeast Iowa are INA consortium in which student is attached to one school but can take classes in all three. Check list if colleges by state to identify very good liberal arts colleges. Also look for a list of Jesuit schools for their small schools. Wonderful small sscools are scattered all over the country.
I looked at colleges that were listed here and I doubt they are as rigorous as swarthmore and reed. Some of them like Grinnell and Davidson are might be, but they are also recognized colleges. So far, from colleges listed here only furman and wooster seem to fit the bill. I have one friend who swears that presbyterian college is more rigorous than any other lac in the country. But also, it’s his alma mater, so I don’t know… Any ideas?
By rigorous I mean, frequency and amount of homework and depth and speed of the program.
Also, this might surprise you, but outside of US Deep Springs is more known than Reed, Swarthmore and W&M.