Looking for free, clear, computer analysis?

I've been downloading free chess engines like rybka 2.2 and stuff but I'm trying to find software that makes it clear when I make a tactical/positional error, and possibly explain a better move.

I've heard people mention computers marking their moves with "?" and "!" and things of that nature.

You get my point, how do I go about getting this?

For free?! Tell me when you find something.

you can run computer analysis on games you play on chess.com, I am not sure what membership you need for that feature..

Something like an older version of ChessMaster would be overkill (and it's around $10 for the "10th edition", but I bought that several years ago, by now it may be in cereal boxes).

It'll analyze games in great depth, provide you with a detailed 'annotated (!!, ?! etc.)' pgn score, along with plain english commentary (violates 'developing the center') for each move that's even somewhat straying from sound chess principles.

sftac

ps.  I suspect this site provides analysis for those willing to pay for premium memberships (~$28-$99/year).