As far as .zip files, just double-click them. You can also create zip files by right clicking (ctrl-click) on an item and selecting Compress...
For rar files I use UnRarX. It's free.
There might also be some tool out there that does all the formats together, but I haven't needed that.
Also, notice that the built-in zip decompressor won't do zip files with passwords. You need to either use the command-line for that or another tool.
EDIT: Here's a free tool that will do pretty much all formats (Rar, Zip, 7z, etc), including archives with passwords: Zipeg

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