Often you will find yourself in a situation where you can upload arbitrary content to a web server.
If the webserver accepts dynamic content (e.g. ASP, PHP, EXE, PL, etc) then you may want to upload a "backdoor shell" to provide a web based GUI for the command line.
None - however keep in mind the following:
1) The backdoor shell may be trojaned. Read the code FIRST!
2) Don't leave it there for too long, as someone else may find it or worse - Google may index it!