MS Exchange alternatives
Now that the best software candidates have been shared above, it's time to decide which one is the best fit for your workplace. My first suggestion is to accept reality. If your company is bent on sticking with MS Exchange, you might be forced to simply rely on OWA. While it may lack all of the functionality you'd find in Outlook, at least you know that OWA will work cross platform.
So what about using the options above that support Exchange? Well, the problem is that none of them do so with Microsoft's blessing. Any Exchange support enjoyed by the end user is happenstance and luck thanks to Microsoft's closed source approach. Despite some great developer efforts with the applications above, none of them offer 'great' MS Exchange support. Denying this is just silly.
If at all possible, look to groupware solutions that can be locally controlled. Or if needed, consider Google or a hosted groupware solution not bearing the Microsoft logo. For example, a solid recommendation would be EGroupware. EGroupware works nicely with any of the applications above. If, however, you're comfortable with a Google hosted option, it would be worth mentioning that it'll make your life easier. Even though many companies aren't comfortable with Google hosting their data, it's a viable option to be considered. Best of all, like with EGroupware, it's going to work smoothly with all of the options above.
And the winner is?
Assuming migrating to Google or EGroupware is viable, which software solution is best? For the enterprise space, I'd avoid relying on a Thunderbird/Lightning combination.
While it's suitable for those who work at home, in the workplace it's really not a good solution. My concern with Thunderbird isn't a lack of support, rather the fact that each update to the client puts the add-ons at risk of not working. This isn't usually a problem with the other PIM clients. Well, at least when you're not trying to connect to Exchange, that is.
I'm also a fan of using the client-side software that is designed to integrate smoothly into your desktop environment. So for KDE users, I recommend Kontact. And for those who run Gnome, consider using Evolution. And for those wondering, yes, Ubuntu dropping Evolution was indeed a really stupid if they're looking to attract enterprise users. Thunderbird is a toy at best, a bundle of patched software at worst, thanks to its add-ons.
The final piece to the Outlook alternatives puzzle is looking into how to best handle syncing to your smartphones. Blackberry, iPhone, Android – no matter how you cut it, you need to keep your data in sync. And while there are a few different localized options that allow you to sync your devices up with your smartphone, the most reliable method is -- and will always be -- over the air (OTA).
Whether you opt for a Google-based solution or one from EGroupware, both options work great OTA and will work well with Android and iOS devices. It's also possible to sync with Blackberry, though I've heard it can produce mixed results.
If I had to recommend one single option, I'd be inclined to recommend going with EGroupware. While both support OTA options for your mobile device, EGroupware offers a vastly more professional appearance and more consistent user experience. Unlike Google Apps, which sends you from page to page, EGroupware provides everything in one area. Everything is tabbed and easy to find. Plus, EGroupware offers additional functionality not found with Google Apps, such as a company news page and a project tracking system.
If your company is looking to dump Outlook once and for all, the applications above will provide you with a solid jumping off point. Be sure to test out each application to see which one is the best fit for your needs. All the software above have different approaches to functionality and UI layout -- there is no perfect fit for everyone.
The last thing to consider is this: if you're dumping Outlook, take this as an opportunity to dump Exchange as well. Even if you find that the localized software selections above aren't good enough, browser access for Google Apps and EGroupware work really well in any modern browser. Unlike MS OWA, you won't be tied down to a limited version of the software if you're not using Windows.