Looking through my acquistions over recent years, here are a few recommendations:
Tamron 24mm f/2.5 -- this lens is sharper than the Nikkor AIs 24mm f/2.8. It can often be found on eBay for cheap. I paid $40 for mine a few years ago.
I agree about the usefulness of 135mm lenses. One of the sharpest I've ever used was a Nikkor 135mm f/3.5 AI. These can often be picked up for cheap and are really worth the price. Another 135 to look for is the Vivitar 135mm f/2.8 CF (CF stands for close focus). This lens, which focuses down to 1:2 is actually a macro -- and a very sharp one. It has recently been "discovered" and often goes for high prices now. Still, you might luck out and find one for cheap. I found mine in a local camera shop's "bargain bin" of old lenses. Paid $5 for it. Woot!
The old pre-AI Nikkor 200mm f/4 is an excellent lens, but usually sells for cheap on eBay. Sometimes you might luck into one that's been AI'd -- even by Nikon -- for no difference in price.
One zoom that can often be found for cheap on eBay, and one I recommend highly, is the Komine-made Vivitar Series 1 28-90mm f/2.8-3.5. This is a very sharp lens, easily equivalent to primes. The later S1 28-105 was made by Cosina and isn't quite as good, so I recommend the original.
Now this depends on your definition of "bargain." But for me, it definitely was. I did some research on the Tamron 80-200mm f/2.8 LD lens and determined that it is the equivalent to the Nikkor AIs 180mm f/2.8 ED in terms of sharpness and contrast. I found one in "BGN" condition (zoom collar was very slightly loose) on KEH for $225. So I bought it and I love it! And it really is as sharp as the Nikkor 180 ED (I used to own one).
This is also a Tamron, and also depends on your definition of "bargain," but once again, for me, it definitely was. The Tamron 300mm f/2.8 LD-IF usually sells on eBay for $700 and up. I found one at auction with an opening bid of $500. Nobody was bidding on it because the seller revealed that it had a bit of fungus behind the front element. Well, I've cleaned fungus from lenses before, so I saw this as a buying opportunity. Sure enough, there were no other bids, and I got it for the opening bid amount. This is a great lens -- equivalent to the much more expensive Nikon 300/2.8 EDIF.