RapidSwitch are apparently one of the largest dedicated server hosts in the UK. Colocation space from £34/mth, dedicated servers from £49/mth. Build your own spec, pay for what you need. Unlimited bandwidth deals available.
Hetzner offer very, very cheap dedicated servers. AMD Dual Core, 2Gb RAM, twin 400Gb SATA disks, from €49 / month plus €99 setup. Raid controllers optional at €10/month. They also appear to run on green power.
United Hosting offer both UK and US based, fully managed servers. They start from £200 / $300 per month. Recommended by Henry Dillon.
Linode are a linux virtual server hosting company. Jeff Waugh (jdub) in Sydney uses them. Fairly cheap to reasonably expensive.
A hosting provider recommended by Happy Cogs (who designed the WordPress 2.5 admin panel). $20/month for shared hosting (1000 domains, 100Gb, 1Tb) , $50-$150/mth for virtual, $750/mth for a balls out dedicated server. Looks *very* good.
ServerBeach are a hosting company recommended by Tailrank's CEO Kevin Burton. Their servers start from $75/month.
The internet hosting company used by BeWelcome.org as at August 2007.