This Monday Google (https://web.archive.org/web/20120611101530/http://googleblog.blogspot.com/2007/09/our-feature-presentation.html?) officially announced Presently, their web-based application for presentations, a.k.a. Microsoft Powerpoint clone.
Although it is possible to create presentations from scratch using Presently, most users will probably use it to upload their finished (or almost finished) Microsoft Powerpoint presentations (there is a 10 MB size limit). The presentations can then be shared either globally or to a restricted list of colleagues.
There are other online tools that can do similar things, most notably Zoho Show (https://web.archive.org/web/20120611101530/http://show.zoho.com/?) and Slideshare (https://web.archive.org/web/20120611101530/http://www.slideshare.net/?). Zoho Show is similar to Google Presently, but Slideshare doesn't allow online editing of your presentation and currently you can't limit access to your presentation to just the people you want to.
Google Presently (and Zoho Show) are a great way to share your presentations for a lab meeting or conference. Email is just not the best tool for this, especially since presentations are usually large files. Did I mention that using either Google Presently and Zoho Show is free?