There has been a couple questions here regarding alternatives to certain programs, and I'm sure as more people start using Ubuntu, and join this site, there will be more people looking for alternatives to programs they used in there previous operating system.
Therefore I figured I start a thread to list different sites that list alternatives to programs.
(Please just post one link per answer).
LinuxAlt - The Linux Alternative Project
From the website:
The site has a very lengthy list of Windows programs and their Linux alternatives. The website is apparently still under active development.
osalt - open source as alternative
From website:
Just like the other sites suggested it gives alternatives to proprietary programs.
The Linux App Finder's Alternatives page
This website boasts a huge list of software and a search feature. Plus any registered user can add their own alternatives.
alternativeTo
From website:
This site has a sizable list of alternatives for a sizable number of programs.
I always stated my disagreement with such questions.
If you're looking for an app and ask other people for suggestions, never ask it like "please suggest me an alternative for app X". For somebody who never used this particular app X, your question has 0 (zero) informational load.
It is much better to ask for an app that has a given set of features (and enumerate the features), or just state your final goal and let people suggest apps you could use to achieve it.
sudo apt-get install synaptic
Keyword search every package available from the apt-get repositories as well as any ppa installed on your system.