I like tools that will perform more than one task, Notepad++ is a great notepad replacement and has a Document Monitor plugin (installs with standard msi) that works great. It also is portable so you can have it on a thumb drive for use anywhere.
For a command line option, PowerShell (which is really a new command line) has a great feature already mentioned.
Get-Content someFile.txt -wait
But you can also filter at the command line using a regular expression
Get-Content web.log -wait | where { $_ -match "ERROR" }