Sometimes Hackage isn't available, in which case you can use one of the mirrors.

Usually you can just replace the remote-repo line in Cabal's config, which is located in:

• ~/.cabal/config on Linux (and probably OS X)
• %APPDATA%\cabal\config on Windows

Recommendations

If you need documentation, Stackage is your only option. Otherwise you might choose the mirror by speed. The speeds of these Hackage mirrors are as follows for various countries (tested by downloading the tarball for lens-4.9):

                         luite.com   reesd.com   fpcomplete.com  |  Hackage
                                                                 |
Netherlands, Amsterdam     144 ms     126 ms *      229 ms       |  200 ms
Belarus, Minsk             700 ms    1116 ms        667 ms *     |  270 ms
UK, Bristol                195 ms      75 ms *      277 ms       |  160 ms
UK, Edinburgh             1114 ms     837 ms        360 ms *     |  180 ms
USA, San Francisco        5373 ms    1455 ms        445 ms *     |  210 ms
USA, New York              822 ms     719 ms         77 ms *     |  165 ms

* = fastest for this location

So, if Hackage isn't down, stick to Hackage (because it's consistently fast), and otherwise you should probably choose FPComplete's mirror unless you live in Bristol.