As of June 23, 2010 Oracle Application Express (APEX) 4.0 is now available on the Oracle Technology Network (OTN) here. This is our (the Oracle APEX development team's) most significant release since Oracle APEX was first released in 2004 (as HTML DB). Oracle APEX is a unique product (actually it's a no cost feature of the Oracle Database), in that it runs within an Oracle database, and is capable of providing multi tenant / self service application development.
Here is how it works, you simply navigate your browser (ff, ie, safari, chrome) to an "instance" of Oracle APEX (which is simply an URL, for example our kick-the-tires free public service is http://apex.oracle.com) and sign up for a "workspace". Once you have an Oracle Application express workspace you can start exposing your Oracle database data on the web by creating an "application". If you don't already have Oracle database tables created, you can create, load and manage your database objects using the Oracle APEX SQL workshop.
You can create simple applications that report on database data or more complex applications such as the Oracle Store. You create these applications using declarative programming (wizards, property sheets) all from your web browser. For example you can click on the APEX Application Builder icon, then on the create application button and build an application using a simple wizard. Then you can add report pages, form pages, integrate web services, create charts, calendars, etc, link all these pages together using tabs, page branching, and list controls giving database developers with a working knowledge of SQL and relational databases the ability to create useful multi user web 2.0 application in minutes. Even if you haven't "programmed" in years but understand databases you can quickly master the basics of Oracle APEX.
You can install Oracle APEX on your laptop, including a MAC by running a native db or using a database within a VM. You can install APEX 4.0 into an Oracle database 10.2 (or better) in your organization, or you can use an instance hosted by Oracle or another hosting partner. You can even run APEX in on the Amazon cloud using the Oracle AMI, EC2 and EBS. Since the client for development and runtime is a browser, you can develop your application from any browser that has connectivity to your development instance, this includes the Apple iPad, Linux OS, MS Windows, etc. As of APEX 4.0 we do not support IE 6 (because it's a tad buggy with modern stuff), but you can create applications that are IE 6 compatible. Personally I work almost exclusively in Safari, so I can "launch" my development too, login, and start developing and tweaking an application in seconds from just about anywhere.
You can read more, much more, about APEX on our OTN pages http://otn.orcle.com/apex.