Connectify is an application for Windows 7 that effectively turns any Windows 7 computer into a virtual WiFi hotspot. Is there any feature like this for the Mac ?
Use Internet Sharing. It's in System Preferences -> Sharing pane -> Internet Sharing option. Before enabling it, just select the Internet Sharing section, select which interface you're connected to the internet via from the "Share your connection from:" pop-up menu, then enable the Airport interface under "To computers using:". Then click Airport Options, and choose a wireless network name (SSID), channel, and optional WEP password (no WPA support, sorry). Once you've configured all the options you want, start the service by enabling the "on" box next to the service name in the left column.
Note that you can also share over other interfaces at the same time. For example, if you have an internet connection via a WWAN card, you could share that over wireless and wired ethernet simultaneously.