Awful Green Things is great. Even better if you have two sets and a GM and play with your opponent 'hidden'
Powergrid is a little heavier than Peurto Rico but I prefer it but is a classic 'Euro' game. Perhaps my favourite of that type.
Dune (an old out of print game now) is a classic and really good with the right set of players. You need at least 4 and preferably 5 or 6 for it to really work though. Successive win attempts mean that the cards and money should circulate and keep everyone in the game. Titan is excellent with 2 or 3 but too slow (too much down time when others decide to fight out the battles) with more.
I like the 18XX games but they are quite long for games which you often know you are out of early. Same with Civ & Advanced Civ. This is a fault with a lot of the 'older' games. Players are effectively out of the game but you need them to stay playing for a few hours or the game folds. Sound familiar
Paths of Glory and the other card driven war games tend to be very good. They are really taking over the strategic war game market (with the block games). Perhaps because they can be played in between 6-10 hours (and can be played via Vassel & Cyberboard) but have the scope and skill of some of the larger strategic wargames of the past. You know the ones we all have stuck up in the loft that take 10 hours to set up and 100 to play....
RA is a great auction game. But if it is a pure auction action you are after Modern Art is also very good.
Colossal Arena is decent hidden betting game with a fantasy theme. Evo is a great dinosaur evolution game with you bidding on genes to improve you dino (but spending victory points to do so) before the meteor strikes. Primordeal soup is similar but involves you bidding for genes on your amoeba and drifting around looking for the right coloured mixture of food to eat while cr**ing out your own colour that your opponents like to feed on
For lighter games Carcassonne & Ticket to Ride are good. But I prefer card games like Wyatt Earp, Schnappchen Jagd, Bohnanza, Frank's Zoo and For Sale for lighter quicker games.
Boardgame geek is a great resource for all things boardgame. The link is in many of the above posts. They also have a great system of Geekgold which is used to buy yourself avatars and to exclude adds etc from the site. You get it for contributing to the site - reviews, pictures etc. and you can also gift it to others.