2.4 GHz is Industrial, Scientific, and Medical (ISM) open spectrum, as is 5 ghz.

Users must accept interference from other transmitters operating in the same band and may not cause
interference to the primary users in the band. Primary users are government systems such as airborne radiolocation systems that emit a high EIRP; and Industrial, Scientific, and Medical (ISM) users.

A key advantage of wireless carrier frequency packet radio is that given permissable emmisions, data is able to be transported up to 30 km line of sight.

There is quite a difference in price range between the consumer systems and commercial systems.  There are different standards that make up the 802.11 industry specification, 802.11 b  802.11 a , and 802.11 g.  The 802.11a standard that operates at 5 GHZ, at permissable power levels this frequency has a shorter distance of propogation, 900 Mhz is also a spectrum that is being used to transmit data services, and has the advantage of propogating more easily through non line of sight interfering clutter.
As a general rule, the higher you go in the spectrum, the more likely it is that rain, trees, and other items will interfere with that frequency.

An excellent introduction to wireless signal path analysis, has been put together by Barry McLarnon, VE3JF:
VHF/UHF/Microwave Radio Propagation:  A Primer for Digital Experimenters.

If you wish to learn more about spread spectrum technology, including information about FCC rules, The Tucson Amature Packet Radio Corporation has an article by Steve Bible, N7HPR:

Spread Spectrum - It's not just for breakfast anymore!
 

News

The FCC has decided to increase the frequency range available to unlicensed wireless
network devices, boosting the band by 80%. However, it's unclear that the boost
will provide much in the way of improved performance for consumers. Rather, the
move is more oriented towards bringing the US and Europe together spectrum-wise.
It's hoped that doing so will lower the overall cost of wireless technology, since the
same technologies could now be used in both places.

Security

A disadvantage is that the system is to some degree vulnerable to determined efforts to circumvent secutity.

AirSnort and WEPCrack are wireless LAN (WLAN) tools that recover encryption keys.  AirSnort operates by passively monitoring transmissions, computing the encryption key when enough packets have been gathered.

802.11b, using the Wired Equivalent Protocol (WEP), is crippled with numerous security flaws. Most damning of these is the weakness described in "Weaknesses in the Key Scheduling Algorithm of RC4 " by Scott Fluhrer, Itsik Mantin and Adi Shamir.

 For more information on wireless security see this Ars Technica Black paper.

Government Initiatives

Industry Canada has launched a pilot project, Broadband for Rural and Northern Development, to research the viability of supporting rural access to broadband through the project.  Chelsea, Quebec, is one of the first communities to be funded to develop a buisness plan, to implement a project, this has been accomplished through as local internet based organization, Resort to Work.

Antenna Designs
 

Martti Palomaki has a number of well presented antenna designs and comparisoms.

Greg's Wireless Networking Info Page has a design for a circular wave guide, as wells a spreadsheet that allows a modification of the related parameters or the design. It is also possible to find links to the Amature Radio Association there.

Jason Heckler's web site features a design for a helix antenna.

For fairly detailled information on microwave antenna design there is The W1GHZ Online Microwave Antenna Book.

Bridges and Repeaters

Linksys, Dlink, offer the software to enable an acess point to perform as a repeating bridge.

mesh ap

Locust World's primary interest is simply in providing the enabling technology to Mesh Networking and .Community Wireless to work

There are no plans to commercialize the core LocustWorld MeshAP device, this will always be available to download from the website as an open product. We believe the best way forward for this techonology is through open standards and open products. Without these, the technology will not gain public support or be subject to the testing and peer review necessary for a world class product.

open ap, is an open source project for reconfiguring access points to perform as repeaters.
 

Community Wireless

CommunityWireless.org is an umbrella organization - representing the needs of the emerging community networks

Seattle Wireless is a not-for-profit effort to develop a wireless broadband community network in Seattle.

bay area wireless user group

AZWarDriving.com was created to supply the Arizona WiFi enthusiast community with a central location to gather and share ideas.

Based in Green Bay, Wisconsin, Green Bay Professional Packet Radio is a community based, advanced wireless communication research team. At this site you will find detailed electronics signal processing designs, Federal Communications Commission rules, and useful radio path analysis tools.

Wireless Power Transfer

Electromagnetic energy is carried by microwaves, and this energy can be gathered by anteena. More information on this will be included soon, for the moment  information on a project using a magnetron from a microwave oven is available at the  Green Bay Professional Packet Radio site.