Sybase official sites
The main entry to all Sybase Linux information is at http://www.sybase.com/linux.
Direct links to the ASE-related parts are at http://www.sybase.com/ase_linux.
From the main http://www.sybase.com website, there are plenty of useful links, you can also go directly to the support site, http://www.sybase.com/support/. You may need to generate a login to get access to all the unrestricted information - registered support users get further access to their own account details in order to directly log and trace their issues on the web.
White papers etc are located at http://www.sybase.com/support/techdocs/.
Manuals are at http://www.sybase.com/manuals/, also known as "SyBooks". Choose "Adaptive Server Enterprise" as the product or go directly to http://sybooks.sybase.com/as.html. Of particular interest is perhaps the "Transact-SQL User's Guide", the "Administrator's Guide" and "Performance and Tuning Guide".
CodeXchange is a relatively new section of the Sybase site, intended for sharing code and tools. http://www.codexchange.sybase.com
Unofficial sites
The International Sybase Users Group is at http://www.isug.com/, and they host the unofficial FAQ at http://www.isug.com/Sybase_FAQ/.
Lots of stuff you will need to know sooner or later, including several printed reference books, have been published by Rob Verschoor and can either be downloaded or ordered at http://www.sypron.nl. Plenty of good links are maintained at http://www.sypron.nl/syblinks.html.
The author of SybPerl, Michael Peppler, has written an ASE on Linux FAQ which is at http://www.mbay.net/~mpeppler/linux.html. The same pages of course host SybPerl and various other utilities and information.
Ed Barlow has written a set of very useful stored procedures for ASE administration, http://www.edbarlow.com.
Third Party Utilities
Many people prefer a tool known as "sqsh" to replace the standard isql utility provided by Sybase. You can download this from http://www.sqsh.org.
If you connect from Java over JDBC using Sybase's jConnect driver, you may want to use iSQL-Viewer as a GUI replacement for isql or jisql. Note that result sets from Sybase stored procedures are not visible in the current release version (2.1.5), download the core classes from CVS until the next version is released. The author has corrected the problem and the versions checked into CVS handle multiple result sets returned by a query. Available at http://isql.sourceforge.net.
News Groups
There is a public USENET discussion group named comp.databases.sybase, but you can also find lots of official Sybase news groups at the news server forums.sybase.com.
Mailing lists
You can sign up for information about new white papers, patches, announcements etc. when you register at http://www.sybase.com. This will only send you updates on the topics you sign up for, Sybase is very strict on avoiding spam and unwanted advertising.
There is an ASE Linux mailing list hosted at http://mathforum.org/epigone/ase-linux-list.
There is also the SYBASE-L mailing list, see info from the URL http://www.lsoft.com/scripts/wl.exe?SL1=SYBASE-L&H=LISTSERV.UCSB.EDU.