As a GIS developer, my design skills are rather limited. Are there any links, books or resources, that you would suggest to make my online maps more pleasing and beautiful?
The CartoTalk forum is a good resource for map design and critique. Gretchen Peterson has written a good map design fundamentals book, very accessible and to the point : http://www.gretchenpeterson.com/
For specifics about online mapping applications, there's geography 2.0, already mentionned, but you can learn by example (many lists are available on the web) and read some Web Graphic Design articles : http://www.smashingmagazine.com/2010/04/06/maps-in-modern-web-design/
A dedicated blog : http://giswebmaps.blogspot.com/
1) Designing Better Maps: A Guide for GIS Users demystifies the basics of good cartography, walking readers through layout design, scales, north arrows, projections, color selection, font choices, and symbol placement. Recognizing the need for integration with other publishing and design programs, the text also covers various export options, all of which lead to the creation of publication-worthy maps. Designing Better Maps includes an appendix describing the author's popular ColorBrewer application, an online color selection tool.
2) Designed Maps: A Sourcebook for GIS Users is a companion to the highly successful Designing Better Maps and offers a graphics-intensive presentation of published maps, providing cartographic examples that GIS users can adapt for their own needs. Each chapter characterizes a common design decision and includes a demonstration map annotated with specific information needed to reproduce the design such as text fonts; sizes and styles; line weights, colors, and patterns; marker symbol fonts, sizes, and colors; and fill colors and patterns. Visual hierarchies and the purpose of each map are considered with the audience in mind, drawing a clear connection between intent and design. Designed Maps also includes a valuable task index that explains what ArcGIS® 9 tools to use for desired cartographic effects. From experienced cartographers to those who make GIS maps only occasionally, all GIS users will find this book to be an indispensable resource.
Although there isn't an emphasis on web/online maps, the techniques do transfer, especially if you are tiling/caching your maps as opposed to creating dynamic maps.
Note you can probably find these books in University libraries, and they are almost certainly cheaper when purchased via Amazon.com, etc
http://cartography2.org Is an good beginning
The Esri Mapping Center is an amazing resource. http://mappingcenter.esri.com/index.cfm?fa=home.welcome I learn a new technique from virtually every blog post, and "Ask a cartographer" is a huge archive of individual questions. It takes a while to explore the site and find everything, but try out every tab.
For your particular issue though, go to the Web Mapping tag and the Publishing tag: http://blogs.esri.com/Support/blogs/mappingcenter/archive/tags/Web+Mapping/default.aspx http://blogs.esri.com/Support/blogs/mappingcenter/archive/tags/Publishing/default.aspx While Web Mapping is helpful, the Publishing tag also contains a few extra techniques (especially for map cache production).
There is supposed to be an upcoming post from Dr. A Jon "Dr K" Kimmerling of Oregon State on basic web map design. Having know Dr K and his work personally, I have some high expectations from that.