The beauty of I-orp was that you could get some kind of answer very quickly, all of the alternatives have a harder learning curve. I-orp could do holistic math over your lifetime, whereas for the alternatives, you will have to set up things like a Roth Conversion plan and test whether each step of Conversion helped or hurt.
If they only question you are trying to answer is "do I have enough to retire?", the FireCalc program is nice in that it gives you historical analysis of how you might have fared in past markets. That is a great starting point as it gives you a feel for your overall risk level. It's not a planning program so does not address questions like Roth Conversions, account withdrawal order, or calculate any taxes.
I use Pralana Gold (paid Excel sheet), it is very powerful and flexible, they plan to release it as a web app by summer. It has static projections (like I-orp) and historical returns and Monte Carlo are available. You can use a variety of withdrawal methods, from setting your expenses to consumption smoothing (like I-orp), to amortization based, variable percentage, etc. It can handle many different kinds of variable cash flows and accounts.
For Roth Conversion planning, for each year, you select whose account to convert first and the ordinary income bracket or other common limits like IRMAA tiers, LTCG tax phase-in or ACA FPL and the program instantly tells you the impact on your final estate value.
Pralana Gold's tax package is the best of the available programs, you tell it about your ordinary and qualified dividend vs. retained capital gains and carryover gains and losses and it will track your average unrealized gains and losses. When you need to sell to raise cash, it will apply taxes. This is important as one of the key brakes on doing Roth Conversions is the need to sell something to pay taxes, other programs miss the resulting capital gains taxes and so may overstate the desirability of Roth Conversions.
A free alternative that is fairly powerful is the bogleheads.org Retiree Portfolio Model (RPM) spreadsheet, available at their wiki. I found hard to learn and annoying that I had to write formulas to take into account things like variable income or expenses. It has a Monte Carlo optional program, but I never tried that. From what I know, RPM and Pralana Gold are the only ones to support a "tax efficient" asset allocation, where you put bonds in your IRA, stocks in Roth & taxable. That's the strategy many/most people use, and since it already helps on taxes, it reduces the urgency of Roth Conversions, so it's quite important that your model supports it.
I have seen folks use a paid web app called NewRetirement, though I've heard it described as much less powerful than Pralana Gold.
There is a thread somewhere on this site about folks using the free Fidelity tool, though I don't know anything about it.
Expect to spend time learning the tool and be inquisitive about the results before taking action, maybe try another tool and run your circumstances by the folks here. You do not want to miss something important.
There are one-time-fee based planners you can hire instead of doing the work yourself, but they cost several thousands and need to be paid each time you re-plan. Stay away from the Asset-Under-Management fee types, their fees, plus high cost investments and often self-serving advice will cost you at least tens of thousands per year.
