Insert a tab when you type ctrl-space or other custom keystroke.
Control Space to Indent acts as a Tab key for webmail and other online forms. When you press a custom keystroke (control-space by default), it inserts a customizable amount (default 8) of spaces into the email, textarea or input you're typing in.
Version 0.52 includes a bugfix that would prevent using the spacebar from checking checkboxes. This previously worked with gmail but they've changed their code, so a gmail-compatible fix is upcoming.