I think it's important to know exactly what you want to achieve. It's rare for a person to need to use their own machine as an access point. If the need is to detect devices "speaking" 802.11, then a tool like kismet is really the way to (Linux only). If however, you want to be able to process 802.11 association requests, then there is an app called hostap (available in the debian / ubuntu repositories by default) that is a configurable 802.11 access point. It puts your network card into "infrastructure" mode, flipping that bit in the 802.11 frames and will send out 802.11 beacon frames like an AP. However, alone it's not 100% of the funtionality of an AP you buy at a big box retailer. To achieve that level of functionality, you also need to install and configure a DHCP server, and probably some routing / firewalling funtionality as well.

As a side note, a fun project to run hostap, a DHCP server, and Squid proxy with a model that passes all images through image magick to either make all the images fuzzy, or to turn all the images upside down. Then rename your SSID to match a commonly used ESSID like "Linksys" or something so people to connect to it.

Another fun / useful project is to make a "myfi" of your own. I've done this using hostap, a DHCP server, a DNS cache daemon, a proxy server, some quick rules in iptables, and my Sprint EVDO card w/ unlimitted data service. I thus turn my little netbook into a hotspot for use just about anywhere I can get a signal on Sprint's network.