Recently I added a webpart to a page in Sharepoint with some bad code in it. As a result I wasn't able to load the page anymore. The only thing I had to do was remove the webpart, so I found this little trick. If you add ?Contents=1 to the end of the URL of the page containing the webpart it displays the Webpart Page Maintenance page. Here you can remove the bad webpart.

An example of a URL: http://moss/default.aspx?contents=1

After that I decided to collect some tricks and add them here,  so here is a small list of links to useful Sharepoint resources.
Sharepoint 2007 URL Quick List, containing a list of commonly used Sharepoint URLs (like the one above).
This one was very useful for hiding fields in a form based on the group a user was (not) in. Uses jQuery and some other JavaScript files. Very cool.
Form settings is a nice feature to completely hide certain fields in forms, without writing code. Integrates with Sharepoint nicely.
When you create webpartpages the navigation on the left side is gone. Using Sharepoint Designer this can easily be fixed.
Very good post on customizing the results of people search in MOSS.
If you have to migrate data from SQL Server to Sharepoint lists or vice versa and you can use Integration Services, this will be very useful. It adds a Sharepoint list source and destination to the toolbox of Business Intelligence Development Studio.
Everything you need to know about Business Data Catalog. Great starting guide.
Very good starting guide on using Reporting Services in Sharepoint.