An overview of some devices you could use and how to configure them:

General:
A large number of SIP/VoIP and Skype phones have emerged onto the market recently and all provide more or less the same possibilities; Wherever you see a Skype phone be sure to read the technical specs to check if it (also) uses SIP/VoIP or is limited to Skype only, but usually any phone can be setup for general SIP/VoIP services.
You should also check offered packaged deals and subscriptions by specific providers (usually giving you a good deal for the device, but be aware: often these devices are password locked! => How2unlock )

Selecting what type of phone you want to use is all a personal matter.
The big question is: do you want it integrally linked to your PC (being up and running) or not.
Stand alone (always on) functioning requires an ATA to be installed, otherwise it is all up to you ;-)


General description of common setup parameters => How2setup
We provide you with samples of set-ups below; note that the devices mentioned do not specifically have our approval or support;
The information given is for sample purposes only, and attempts to give you some guidance in your further choices.


Overview of VoIP devices on the market: www.alibaba.com/VoIP-products
Wide range of VoIP devices from D-Link: www.dlink.com/VoIP-products

Softphones:
One of many softphones: www.sjlabs.com/sjp.html => How2setup
See for further featured Softphones under "Info/Links".

ATAs (Analogue Terminal Adaptors):
D-Link DVG-2001s: www.dlink.com/DVG-2001s => How2setup
Grandstream ATAs: http://www.grandstream.com/ata.html
Linksys PAP2: www.linksys.com/VoIP/PAP2.htm => How2setup
NetComm V100: www.netcomm.com.au/VoIP/V100.php

USB Phones:
US Robotics: http://www.usr.com/voip-product.asp?sku=USR9600
(for our mates down-under: the DSE usb phone is this same device ;-)
i-rocks: http://www.i-rocks.com/IR-2500.htm
Too many USB phones are offered on the market to make any further recommendations.

WiFi Phones:
D-Link V-Click: http://www.dlink.com/products/?sec=1&pid=529
Nokia e70: http://www.nokia-asia.com/nokia/0,6771,83850,00.html
(Note that the Nokia e-series does not work behind routers, eg: at home ! )