Tanaza allows you manage inexpensive WiFi routers over the cloud. Tanaza Powered APs, equipped with their freely available Tanaza firmware, provides Captive Portal functionality, using external Splash Pages. Some developers use it to provide Social Media hotspots to restaurants, pubs and retail stores (demo).
You can sign up for a FREE account and get unlimited service for 2 Access Points. More than two APs require only a modest annual subscription fee. You can centralize management of dozens of APs in different locations.
Now Tanaza says they will be able to cloud manage any OpenWRT-compatible device, shortly. You can flash a number of inexpensive WiFi routers with OpenWRT firmware. OpenWrt (Wikipedia) is a full-featured, easily modifiable operating system for your router. Then you can cloud manage the lot of them. Even mix and match brands.
Tanaza says they have an internal version of the Tanaza Agent that allows to cloud manage OpenWRT devices through the cloud. The project is currently in private alfa and will be released publicly as it goes beta.
They currently provide free firmware for a variety of Ubiquiti access points. You can download Tanaza firmware and flash your device to get a Tanaza Powered Access Point, adding cloud management and remote monitoring capabilities to it.