FixMyTransport is a newly launched site that lets you report problems with public transport. Your gripe is then escalated to the appropriate authority. Progress is reported beneath your original complaint or suggestion.
The site has an impressive and clean interface, with clear instructions on the first page. You can report one-off incidents (e.g. vandalised ticket machine), or launch campaigns for more ambitious improvements (more station lighting, better information boards...). For any issue, you can use the site to rally support and keep track of correspondence.
Like all such people-power sites, FixMyTransport can only be truly effective once its user-base grows enough to give campaigns any clout. However, if managed right, the site may score early success by alerting authorities to smaller, easily fixed issues (broken seat at Twickenham bus stop).
The site is in public beta, so there's a fair bit of development to come, including the all-important mobile interface. We'd also suggest adding a search by transport type (it'd be good to browse through all issues related to cycle-hire, say, or DLR).
So if you find yourself grumbling about your local station or service, give the site a go and see if it helps turn things around.