Samsung actually created a nice mobile Wifi Access Point application for the Galaxy S phones but T-Mobile in their infinite wisdom had it removed (to make room for highly useful tools like Avatar no doubt). Well now you can install it and create an Infrastructure Wifi Network. Again, this mod requires you to install the zip file using Clockwork Mod.
Note: If you are using a custom ROM rather than the original ROM, this may not work.
1. Download the zip file (download link)
2. Copy zip file to phone
3. Use ROM Manager to reboot into Recovery Mode
4. Install the Zip from from SD Card
5. Reboot
To use this, go to Settings –> Wireless and Network –> Mobile AP Settings
You can now setup and manage you Wifi hotspot