Update 2010.07.09: wikiHow has a great article on clearing browser cache.
Internet Explorer 6:
Tools (menu) -> Internet Options (menu) ->
Internet Options (control panel) -> General (tab) ->
Temporary Internet Files -> Delete Files (button) ->
OK (button)
Internet Options (control panel) -> General (tab) ->
Temporary Internet Files -> Delete Files (button) ->
OK (button)
Internet Explorer 7:
Tools (menu) -> Delete Browsing History (menu) ->
Temporary Internet Files -> Delete Files (button) -> Yes (button)
Temporary Internet Files -> Delete Files (button) -> Yes (button)
Firefox 2:
Tools (menu) -> Clear Private Data (menu) ->
Cache (checkbox) -> Clear Private Data Now (button)
Cache (checkbox) -> Clear Private Data Now (button)
Firefox 3:
Tools (menu) -> Clear Recent History (menu)
Time Range -> Everything (select menu)
Details -> Cache (checkbox)
OK (button)
Time Range -> Everything (select menu)
Details -> Cache (checkbox)
OK (button)
Safari:
Safari (menu) -> Empty Cache (menu) -> Empty (button)
For more information on web cache: wikipedia.org/Web_cache