It mainly depends on your requirements... You could look for general purpose UML/SysML modelers, or more specific software development frameworks, or even build your own software designer (with tools like Obeo-Sirius, Xtext, Emf...).
If you plan to work with a team in a collaborative way, I would suggest to invest in a commercial tool (or check that you can effectively modify your models in a collaborative way, whatever your final choice).
If you plan to exploit your models with custom extensions (custom code/doc generators, validators, ...), I suggest to consider modeling tools based on the EMF framework (rich emf ecosystem, allowing to implement a lot of things with your models).
Top Vendor specific tools: 1-) Rational Software Architect 2-) Rational Software Modeller 3-) IBM Rapsody 4-) Enterprise Architect
Top opensource tools: 1-)BOUML - an UML tool box BOUML is a free UML 2 tool box allowing you to specify and generate code in C++, Java, Idl, Php and Python 2-)Modelio Open Source Community Modelio is an open source modeling environment. Modelio delivers a broad-focused range of standards-based functionalists for software developers, analysts, designers, business architects and system architects. (GPL) 3-)Papyrus Papyrus is aiming at providing an integrated and user-consumable environment for editing any kind of EMF model and particularly supporting UML and related modeling languages such as SysML and MARTE. (EPL) 3-)Umbrello - The UML Modeller Umbrello UML Modeller is a Unified Modelling Language diagram program for KDE. (GPL) 5-)UML Designer UML Designer provides a set of common diagrams to work with UML 2.5 models. The intent is to provide an easy way to make the transition from UML to domain specific modeling. (EPL) 6-)Umple: Merging Modeling with Programming Umple is technology for Model-Oriented Programming. It adds abstractions such as UML associations, attributes, and state machines to object-oriented programming languages such as Java, PHP and Ruby. (MIT) Best Regards
Astah, formerly known as JUDE (Java and UML Developers' Environment, pronounced Jūdo ، is a UML modeling tool created by Japanese company Change Vision. JUDE received the "Software Product Of The Year 2006" prize, established by Information-Technology Promotion Agency in Japan.