If you install xtrSENSE or xtrROM (or just about any other custom ROM, but you mention 2.1), it will come with a WiFi tether app pre-installed.
Note that this creates ad hoc WiFi connections - they are not the same as hotspot connections, which are more like a wireless router. This is important because devices like the Amazon Kindle and (I think...) the iPod Touch and Wifi-only iPad cannot connect to ad hoc WiFi connections. Laptops should be all set, though.
BTW, with an Eris you cannot even pay Verizon for hotspot service - the Eris does not support it, so it is not available.