Real Studio allows you to write one program and compile without modification for all three platforms.
It isn't free and you do need the Pro version to cross-compile for all three platforms. This means you can test applications on all three platforms but only compile and debug on the platform you purchased a license for.
The Enterprise version allows you install the complier on any of the three platforms with the same license code - and provided you stick to one license per user you can install it on as many machines as you want. This means you can cross compile and debug on any platform you choose.
There is also a web version (though it isn't final yet - but due for release any time now) which allows you to produce self contained compiled web applications. There are/will be restrictions on the PHP setup on the web server (requires FastCGI or sufficient rights to run applications on the server). Note this will be a standalone product but will be included with the Enterprise edition (and it supports iPhone and iPad).
There is a fairly active community and comprehensive documentation and third party books available. The compiler is updated regularly (usually 4+ times a year) on a subscription basis.
You can link it to various database engines, including MySQL.
Try out the 30 day trial - if you want to play more there is a cheaper personal edition (doesn't include all the database features and a few other bits are more limited).
Realsoftware also have an odd upgrade policy - basically if you buy a license for personal you can upgrade to pro or enterprise at any time. Upgrades extend your current subscription by 6 months. The cheapest way to get a long subscription on Pro or Enterprise is to buy as many years as you can afford on Personal and then upgrade - you get all your existing years plus six months with the upgrade which is much
cheaper than renewing every year for the Enterprise package!