Though its title refers to Mountain Lion, it is generally applicable to earlier versions of OS X as well.
Whatever you do, do not download or install any product claiming to magically "clean up" or "speed up" or "optimize" your Mac. Without exception, they will do the opposite. If you already did, you found the problem. Fix it.
If it is only Web browsing that seems to be slower than it used to, that is often the result of such sites becoming ever more demanding of hardware and bandwidth. Short of paying for faster Internet service, buying additional memory, or a faster Mac, there is little you can do to cope with such so-called progress.
If you're experiencing general slowdowns or "freezes" unrelated to Internet activity, the problem may be related to your computer or the software you installed and is something you may be able to fix, or at least improve. Aside from hardware failures and software that you install, including OS upgrades, there is no reason that your computer should not work precisely the same as it did when it was new.
Without a more specific description it is difficult to provide specific guidance. Read the following to determine if something may apply to your concern:
General Mac maintenance: Tips to keep your Mac in top form
General purpose Mac troubleshooting guide: Isolating issues in Mac OS X
Creating a temporary account to isolate user-specific problems: Isolating an issue by using another user account
Identifying resource hogs and other tips: Runaway applications can shorten battery runtime
To identify potential hardware problems: Apple Hardware Test
To resolve startup issues and perform disk maintenance, use Disk Utility.
Safe Mode or "Safe Boot" is a troubleshooting mode that bypasses all third party system extensions and loads only required system components.
Read about it: Starting up in Safe Mode
To repair a potentially corrupt hard disk, so that you may recover its data prior to replacing it, and subsequently reinstall Mac OS X: OS X Recovery (applies only to Lion and later versions of OS X)
Secure empty trash, empty non essential caches, use the stacks as opposed to having 200 icons on your desktop. Also there is a good primer on keeping your Mac in good shape, called .step by step to fix your Mac by ds store.
Hope this helps,
Secure empty trash, empty non essential caches, use the stacks as opposed to having 200 icons on your desktop.
Securely emptying trash has no effect whatsoever on performance compared to emptying the trash normally (except that secure emptying takes a lot longer). Emptying caches is a troubleshooting step only, and should not be done as regular maintenance. (Besides, which caches exactly are "non-essential?" No caches are essential, but they all help improve the performance of the machine.) Finally, having many icons on the desktop is not the performance drag that it used to be years ago. That issue was fixed a while back, when Apple changed the way desktop icons are displayed.