Point them at the wiki
You do have one, don't you? And I'm sure you have a few team members who act as code librarians during the code reviews so that common code can be introduced (either into the company code library or to replace the custom code that the developer has just knocked together).
No? Ah. Well, you've got a problem then.
Depending on how much time you've got, you'll need to follow one of these plans, in order of the level of crisis:
1. Aargh! I'm leaving this afternoon
If the new guy is highly experienced in your problem domain, then just point him at the wiki and show him how to get to the relevant code in your SCM. Buy him a beer, offer your phone number if you're feeling generous and toddle off to your leaving party.
If he's not so experienced, introduce him to other members of the team who should be his primary points of contact to get an idea of how it all fits together. That's about all you've got time for.
2. Got all week, but there's a lot of tidying up
Get your replacement to shadow you for the first day so you can assess their skills and try to get them doing your job as quickly as possible. Let them see how you tackle a problem, and then gradually hand over so that they are solving it and you are acting as a consultant. Build out from the core operations you do every day to the less frequent ones. Make up problems if you can. Pair program if you're able to. Get them to keep track of what you've told them in a wiki or some other networked text-based resource so they can refer to it later, and eventually turn it into proper documentation.
3. It's the junior developer who's taking over
They'll already have the domain knowledge and know how the team works. Start allocating them your tasks and working to build their knowledge about unfamiliar systems and processes. Concentrate on helping them get the basics down pat, with the more advanced stuff coming later as you get time.
Provide basic telephone support
Assuming that you're leaving on good terms with the company, offer to provide them with some telephone support so they know that you're not leaving them in the lurch. If they start ringing at every hour asking bone-headed questions that could be found on the wiki then mention your competitive consultancy rates. Otherwise, the odd call here and there allows them to pick your brains and you to stay in touch with them, which could be handy when the next round of jobs comes up.