Relative merits of different open source natural language generators
In terms of open source NLG components, I'm most familiar with Mumble and FUF/SURGE. They've got both similarities and differences, so it's hard to say which is better...

Mumble:


written in Lisp
EPL license
based on tree-adjoining grammar
focuses on linguistic message planning


FUF/SURGE:


written in Lisp
GPL license
based on functional unification grammar
focuses on syntactic realization


Since it sounds like you're interested in abstractive summarization (which is much harder than traditional extractive summarization), I'd recommend the following academic papers:


Text Generation for Abstractive Summarization
Framework for Abstractive Summarization using Text-to-Text Generation
Towards a Framework for Abstractive Summarization of Multimodal Documents -- full disclosure: I'm the author of this one


Also, consider checking out this textbook to get started: Building Natural Language Generation Systems