Is there any way to block certain sites, eg MacKeeper?

MacKeeper (and other sites) adverts pop up on some video sites which I find very irritating. Is there any way to block specific URLs in Safari?

This occurs both on Mac and iPad. Ideally, I'd like a solution I can directly apply to the iPad.

I would suggest OpenDNS. You can apply it to your entire network using their name servers and block categories or specific URL's. Although the ad might show up as a frame to a 'Website Blocked' page from OpenDNS, you'll probably find it useful (it's free).

My prefered solution is using glimmerblocker which is actually an ad blocker but can be also used to just block specific web sites/URLs.

For iDevices it's a bit more difficult (at least if not jailbroken). Glimmerblocker supports a proxy mode though which allows you to point Mobile Safari to the Mac running Glimmerblocker. Of course this requires both systems to be online.