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Machinarium and The Path have gotten nothing but rave reviews. I've tried The Path -- loved it, though it's not for everyone. Try the demo.

Oh I don't doubt that they have rave reviews. However their longevity is more questionable, and that can't be gauged accurately by a demo.

Replay value? The Path probably has no replay value. It's all about the experience. I suppose you could replay to try and further interpret the meanings, but I didn't feel particularly inclined. If that's important to you, don't buy it. But for $4, I think you'd be nuts to not buy it if only to play once.

Machinarium is one of the most brilliant adventure games I've ever played. There's a justification for FOUR DOLLARS.
I bought it twice. Plus the vinyl soundtrack. Totaling up to about $68.

Actually It's $6.79 for me. Might be regional changes. But oh well, sounds like it is good. Do you know anything about the other games I have mentioned?

The funny part is I've only played through Machinarium once. Haven't booted it up in a year.
I have Bob Came in Pieces and Flotilla, but haven't played them.

Flotilla's pretty good. Nice little basic WEGO space combat game, and a game made to be played in short stints. Your adventure might last 15-20 minutes, then the game's over. Start up next time, and it's a new random galaxy and random opponents and so on. For $3.39 or whatever it is, no brainer.
Zombilution...I suppose it's not bad for that price. Fairly meh overall.
I'd definitely be interested to hear anyone's thoughts on Emberwind or Kingdom for Keflings.

Flottila is a very cool turn based tactical game. The controls and the looks are looks like Homeworld's but in turn based combat.

Personally, I stopped playing Flotilla fairly quickly simply due to the lack of a save game feature. Yes, I know some people can whizz through a play through in 15 to 20 mins, but that's not how I play such games.
And seeing as the dev. doesn't seem to want to add a save feature, despite a fair few requests for one (even with all online scoring disabled), I see no point in ever playing the game again. For me, it was a waste of money.

from the BFG forum

My personal recommendation would be Saira and Bob Came In Pieces. I wasn't really fond of the demo for Saira, but that was mainly due to me playing it on my Netbook and being frustrated with the controls. Other than that, I like the atmosphere of the game - typical in a Nifflas game. Justifies the $4 for me.
Bob Came In Pieces is, well, a wonderful, wonderful little game.
By the way, there's this game called Dungeon Defenders, just added to the list today:
http://www.gamersgate.com/DD-DUDE/dungeon-defenders
Looks like a fun game, and at quite a reasonable price too. It's a mix between an action-RPG and tower defense, and we don't have many games (are there any at all?) that focus on this hybrid genre, so it's worth taking a look at.

Plain Sight is 80% off on Steam

And for $2 I just had to get it. :-)