SharePoint Sandboxed Solutions
There are two types of custom solutions in SharePoint: Full Trust or Farm solutions and Sandboxed or Site Collection solutions. Sandboxed solutions are limited in their capabilities compared to farm solutions, but still, they can solve many business processes. This limited functionality is balanced by limited damage potential – a sandboxed solution cannot bring the farm down – damage, if any, is limited to the site collection where it is deployed. Farm administrators can control and monitor sandboxed solutions. Many custom site definitions (templates), web parts and other features are available as sand boxed solutions.
If you are a site collection administrator and you find a good and safe solution that is available as sandboxed, you can download/unzip the .wsp file and upload it to the Solutions gallery (site settings, galleries, solutions – from the top site). Activate the Solutions tab in the ribbon and then click Upload solution. Browse to your .wsp file and upload it as usual. Once the file is uploaded, click the Activate button on the ribbon. Follow any other instructions from the source (some solutions require activation of some site features).
Notice that SharePoint will allow you to upload farm solutions to the gallery (should not do so, but could by mistake), but you will not be able to activate them. They will just sit there and you can delete them. Activated sandboxed solutions can be deactivated similar to activation and once deactivated, they can be deleted.
You should be careful with sanbboxed solutions even if their potential damage is limited – you could, in theory, wipe out your site collection if you install untrusted solutions. A good place to look for free solutions is Codeplex.