On Tuesday, Google is expected to finally introduce its mobile, open-source Android platform to the world.

Android has attracted widespread interest and support, but it's not alone: Android joins LiMo, Maemo, Symbian and Openmoko in a proliferating set of alternatives to proprietary mobile platforms. The alliances behind each of these, their capabilities, potentials and downsides can be confusing. Here's s short guide from ZDNet.com.au's sister site ZDNet.co.uk on who's doing what, why and how.

What is Android?
Android is a full mobile stack, from operating system up to applications, developed by the Google-led Open Handset Alliance. Although the first Android handset comes from HTC, many manufacturers could release devices based on the system.

What are the pros?
Google's reputation among consumers is enough to pique the interest of a lot of developers, a situation enhanced by the company's deep pockets and its willingness to reward those who come up with what Google thinks will be prized applications.

T-Mobile's co-operation may also make for a good start, as the operator has a fairly broad reach around the world. Also in Android's favour are Google's many other web-based initiatives, such as the Chrome browser and Google Apps, which are likely to find their way onto handsets in interesting ways via Android.

What are the cons?
Getting other operators on board may be tricky. Google does not have much experience in the mobile industry, and its many projects may cause operators to suspect Google wants to take application-based revenue away from them.

Another issue might be Google's use of the Dalvik virtual machine, which is not fully compatible with other JavaScript engines. This could make it more difficult for developers to port their applications over to Android.

The fact that Android is not yet proven in the marketplace could also dissuade time-poor developers from addressing the platform.

Next: LiMo.

What is the LiMo Platform?
The LiMo (Linux Mobile) Foundation is a consortium of players in the mobile industry, ranging from software developers to operators and manufacturers.

The consortium has come up with its own, Linux-based middleware platform, which LiMo has slowly and quietly started building into consumer-level handsets, such as the Motorola Motorokr EM30 (pictured above).

What are the pros?
Because LiMo's platform is essentially middleware, it aims to ensure compatibility across the industry, without taking away operators' ability to put their own, proprietary applications on top. This has led to great acceptance in the operator community, which is, in effect, the driving force behind LiMo.

Again, because LiMo is middleware, it can run on top of various operating systems, making it an attractive option for developers.

What are the cons?
As a rather slow-and-steady initiative, LiMo has not excited the public much. The greater public awareness of rivals like Android and the iPhone could steer developers in those directions.

Also, LiMo has yet to make an impact on the higher-end, more enterprise-friendly handset market. There have been rumblings that this might happen but, for now, LiMo is strictly geared towards phones in the consumer sector.

Next: Symbian.

What is Symbian?
By far the most successful smartphone operating system in the world, Symbian is the product of a UK-based company that was mostly owned by Nokia, until recently.

Nokia has now bought up the bits of Symbian that it did not already own<, and has joined forces with various manufacturers in a bid to combine Symbian with its derivates, UIQ and Series 60, and turn the whole resulting platform into open source. Pictured above is Nokia's E71, one of the most recent phones to use the Symbian-derived Series 60 platform.

What are the pros?
Symbian's massive popularity already makes it the first choice for developers who want to address the widest possible smartphone-user market. The open-source version will, therefore, garner great interest from an established developer community.

Nokia is also the biggest mobile-phone manufacturer in the world, and its leadership of the platform will automatically put the open-source Symbian into a wide variety of handsets.

What are the cons?
Symbian, in its present form, includes a lot of third-party, proprietary code that will need to be stripped out over the next year or two, before the open-source version of the platform makes its debut. Symbian is still working with third parties, and it remains unclear as to how the company will clear this hurdle and still come out with a recognisable, attractive platform.

Next: Maemo.

What is Maemo?
The upcoming, open-source Symbian is far from being Nokia's only mobile, open-source play.

For the last three years or so, the Finnish manufacturer has been behind the Maemo Linux operating system, which it uses for its internet tablets such as the N810 (the WiMax edition of which is pictured above).

What are the pros?
Maemo has garnered a lot of interest from developers who are keen to address what many think will be an exploding market: that of the mobile internet device (MID). A lot of interesting applications have been created for the operating system, and N800-series devices have been the most visible MIDs to date.

Nokia also recently announced that the next generation of Maemo would, for the first time, support cellular connectivity. This will make it possible to create Maemo-toting MIDs that can use, for instance, HSDPA for data connectivity — until now they have only been able to use Wi-Fi.

What are the cons?
Developers have, thus far, been about the only people to buy Maemo-toting devices. The operating system remains, for now, fairly small fry in the mobile, open-source market.

Next: Openmoko.