Tools for design of solar systems using air as the heat transfer media.
Tools for sizing ducts and fans.
Pipe flow pressure drop calculator.
See Also:

Pump and Plumbing Sizing for Solar Water/Space Heating Systems  
Pump and Plumbing Sizing for Solar Water or Space Heating System  This page covers the details of sizing the pump and the plumbing for a solar space or water heating system so that the system efficiently transfers heat from the collector to the storage tank without being excessively large. 
How to Determine Collector Flow Rates  This page takes you through how to determine flow rate for your collector, and what the tradeoffs are ... 
HVAC  
Pipe
Flow Calculations http://www.pipeflowcalculations.com

Nice
set of calculators for pressure drop, airflow calculations, ...

Free Calc  a variety of HVAC
calculators
Air duct pressure drop calculator... Pipe pressure drop calculator...

Some
nicely done and generally easy to use calculators for water pipe and air
duct pressure losses, channel flow, fan laws, ... 
Air Duct Friction Loss Calculator From The Engineering Toolbox www.engineeringtoolbox.com/...

A simple calculator for calculating the pressure losses in air ducts. Useful for sizing ducts and fans. 
Air Friction chart for flex ducts http://www.hartandcooley.com/flex/air_friction_chart.pdf

An easy to use chart to look up
friction losses for flex ducts. I find this one simple chart answers most of my duct loss questions  it even has a correction for bends at the bottom. 
Flex Duct Loses Due to Compression
and Sags... Static Pressure Losses in 6”, 8”, and 10” NonMetallic Flexible Duct... 
Flex ducting is subject to extra
losses compared to rigid metal ducts when it has sags between supports and
when it is not fully stretched out. These extra losses can be
quite larger. This paper measures extra losses for flex ducting with sags and with compressed ducts. 
Stack Effect Ventilation Calculator

Estimates airflow for thermal stacks 
Friction loss in round or
rectangular ducts www.engineeringtoolbox.com/ ... Note: The Engineering Toolbox site has a lot of useful stuff, but also has some very scummy popup windows  so beware. 
A very simple air duct pressure
loss calculator. The duct calculator at the link just above is will handle a wider variety of conditions, but is more complex to use. 
Temperature Drop Calculator for Air Ducts...

This handy calculator estimates the temperature drop in an insulated duct based on inside and outside temperatures, duct flow, duct size, and insulation level. Note that the heat loss that goes with this temperature drop is about: (flow in cfm)(Temp drop in F)(1.08) BTU/hr 
Air Density Calculator http://www.denysschen.com/catalogue/density.asp

Calculates air density for the
temperature and altitude you input.

Reynolds Number Re Calculator... 
A
nice and flexible calculator for Reynodls number  accepts a wide variety
of units and flow conditions. The fluid viscosity inputs that are needed can be looked up on the Engineering Toolbox website. 
Borst Engineering and Construction
Calculators Many Calculators... 
Borst Engineering has a whole raft of calculators  HVAC, solar position, shading, and incident heat, water wheels, pipe sizing, ... 
Pipe Flow  
Copper Pipe Pressure Drop http://www.engineeringtoolbox.com

Handy pressure drop table for
copper pipe. Note: 1 psi = 2.3 ft of head Other pressure drop tools on this site: 
Pipe Pressure Drop Online
Calculator

Nice
online pressure drop calculator. Handles a wide variety of situations. Also figures loss for bends, orifices, nozzles, ... 
Pipe flow and Duct Flow Calculators Mike Rocchetti http://www.connel.com/freeware/index.shtml

Calculators for pressure loss in pipes and ducts, steam tables, and a Psychrometric Calculator 
Hydronic Heaters  Baseboard, ...  
SlantFin Hydronic Baseboard Radiator Output  SlantFin manual on their hydronic baseboard units showing output at lower temps that might be typical of what you would get on efficient solar heating systems. 