Chess Engine

How useful are chess engines for players at my level? I don't have the money to buy one, but I'd like something to catch major positional errors and missed tactics, as well as make accurate suggestions. Are there any engines I can download online for free, or should I not worry about getting one at all?

Note: I don't plan on using one to cheat on any chess sites (namely this one), since my goal is to improve my own chess ability, not to raise an arbitrary number on the internet.

Just google chess engines to download, if you want one. When I watch the masters play on ChessBomb, they have a built in Houdini engine to follow the games with. And I paid attention to the Chess engine championships for a spell, but I don't understand chess engines very well. They make moves that are waaay too deep for me. But then again, I don't like post-mortems either. I just like to figure out things myself

Wiki has a large list of free engines along with their playing strength.

Then there's always this:

http://chessopolis.com/cchess.htm

How do engines work? Do I have to download an accompanying "reader" thingamajig? Or can I simply download the entire thing as a package, open it, and use it like any other software?

Yep, it should come complete with the download.

I have Armageddon (and didn't need anything else).